Asterisk A2billing with VoIP GSM Gateway

The goal is to configure 1 Asterisk server and 1 A2billing -The Server A (Germany)should take a call from (PMX, Cloud Based SIP Trunking ) and transfer it to VoIP GSM Gateway SIM-Box, 20 Channel having prepaid simcards) in Africa -the server shoul be in secure from hacker - The server shall be able to terminate voip calls comming in codecs g729 and g723. Other standards codecs like G722, G726, alow, ulow... shall be supported - Send ussd and ivr to each sim card to topup and credit check. - Display how much credit is available in each sim card. - be able to listen to the channels if they are busy. - If a channel is busy the call shall try automatically the next channel and so on till it finds a free channel. - A2billing shall be used to monitor and configure the system -a Web platform for user to create and topup his credit on A2billing or something else May be my project is not good structured but I hope you understand what I try to do and besser structure it. I want that a customer connect on secure website create a account and topup the credit and call. I did not already buy the GSM gateway may be you can propose me a better solution to achieve my goal. Tell me how long it will take and how much Thanks Raoul